In 2003, the International Diabetes Federation estimated that there were 194 million people worldwide with diabetes. Of these, some 16 million were estimated to be in the United States. Many diabetes sufferers undergo a slow deterioration of the kidneys, a process known as nephropathy. The end stage of nephropathy is kidney failure, or end stage renal disease. Nephropathy and kidney failure can result even when diabetes is controlled with drugs and exercise. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health, diabetes is the most common cause of kidney failure and is responsible for about 40% of the 100,000 cases of kidney failure that develop annually in the U.S. Given the $20 billion annual cost of treating kidney failure in the U.S. alone, reducing nephropathy and kidney failure could significantly reduce the costs of treating this complication of diabetes.
The NIDDK website on diabetes states that, over several years, people with diabetes who are developing kidney disease will have small amounts of the blood protein albumin begin to leak into their urine. At its first stage, this condition is called microalbuminuria. The kidney's filtration function usually remains normal during this period. As the disease progresses, more albumin leaks into the urine. Various names are attached to this interval of the disease, such as overt diabetic nephropathy or macroalbuminuria. As the amount of albumin in the urine increases, filtering function usually begins to drop. The body retains various wastes as filtration falls. Creatinine is one such waste, and a blood test for creatinine can measure the decline in kidney filtration. There are multiple hypotheses about the presence of protein in the urine. One hypothesis is that the protein is an indication of the degree of renal failure. Another is that the leakage of protein from the kidney is not just a symptom of renal failure, but actively contributes to it.
Hypertension is considered a significant factor in the development of nephropathy, and kidney damage tends to increase hypertension. Both a family history of hypertension and the presence of hypertension appear to increase chances of developing kidney disease. Hypertension also accelerates the progress of kidney disease where it already exists. The American Diabetes Association and the National Heart, Lung, and Blood Institute recommend that people with diabetes keep their blood pressure below 130/80. Many people require two or more drugs to control their blood pressure.
Renin, an enzyme released by the kidney, cleaves a circulating substrate known as angiotensinogen. Angiotensinogen is cleaved by renin to form a decapeptide, angiotensin I. Angiotensin I is cleaved by angiotensin-converting enzyme (ACE) to form the octapeptide angiotensin II. Angiotensin II binds to receptors, which results in a number of biological effects, one of which is to cause blood vessels to constrict, increasing blood pressure.
Administration of inhibitors of ACE or of angiotensin receptor blocker (ARB) therefore helps reduce hypertension. Beta blockers, calcium channel blockers, and other blood pressure drugs may also be needed.
Hypertension alone, however, cannot explain nephropathy due to diabetes, since bringing blood pressure down to normal levels will slow development of nephropathy, but will not block it. Progress has been made in slowing the onset and progression of kidney disease in people with diabetes. The NIDDK website on diabetes indicates that drugs used to lower blood pressure can slow the progression of kidney disease significantly, and that both ACE inhibitors and ARBs have proven effective in slowing the progression of kidney disease. One hypothesis is that damage to the glomeruli causes changes to the microcirculation that causes increased sensitivity to angiotensin II.
An example of an effective ACE inhibitor is captopril, which doctors commonly prescribe for treating kidney disease or diabetes. In addition to its ability to lower blood pressure, captopril may directly protect the kidney's glomeruli. ACE inhibitors have lowered proteinuria and slowed deterioration even in diabetic patients who did not have high blood pressure. Further, in persons with type 1 diabetes, ACE inhibitors have been shown to reduce the progression of kidney damage more than other agents that reduce blood pressure by an equal degree. An example of an effective ARB is losartan, which has also been shown to protect kidney function and lower the risk of cardiovascular events.
Methods of determining whether an agent protects against diabetic nephropathy are known in the art, and typically involve determining whether persons administered a putative protective agent release less protein into their urine than persons administered a placebo. See, e.g., Lewis et al. “The effect of angiotensin-converting-enzyme inhibition on diabetic nephropathy The Collaborative Study Group” N Engl J Med 329(20):1456-1462 (1993); Ruggenenti et al., “Chronic proteinuric nephropathies: outcomes and response to treatment in a prospective cohort of 352 patients with different patterns of renal injury,” Am J Kidney Dis. 35(6):1155-1165 (2000); Maschio et al., “Effect of the angiotensin-converting-enzyme inhibitor benazepril on the progression of chronic renal insufficiency. The Angiotensin-Converting-Enzyme Inhibition in Progressive Renal Insufficiency Study Group.” N Engl J Med. 334(15):939-945 (1996); and Hannedouche et al. “Angiotensin converting enzyme inhibition and chronic cyclosporine-induced renal dysfunction in type 1 diabetes,” Nephrol Dial Transplant. 11(4):673-678 (1996).
Additional means of slowing or blocking development of nephropathy are needed. It would be useful to find additional agents or types of agents that can protect the kidney from damage.
Epoxide hydrolases (“EH,” EC 3.3.2.3) are a family of enzymes which hydrolyze a variety of exogenous and endogenous epoxides to their corresponding diols. Epoxide hydrolases have been found in tissues of all mammalian species tested. The highest levels of the enzyme were found in liver and kidney cells (see Wixtrom and Hammock, Pharmacology and Toxicology (Zakim, D. and Vessey, D. A., ed.) 1:1-93, Wiley, New York, 1985).
Four principal EH's are known: leukotriene epoxide hydrolase, cholesterol epoxide hydrolase, microsomal EH (“mEH”), and soluble EH (“sEH,” previously called cytosolic EH). The leukotriene EH acts on leukotriene A4, whereas the cholesterol EH hydrates compounds related to the 5,6-epoxide of cholesterol (Nashed, N. T., et al., Arch. Biochem. Biophysics., 241:149-162, 1985; Finley, B. and B. D. Hammock, Biochem. Pharmacol., 37:3169-3175, 1988).
The microsomal epoxide hydrolase metabolizes monosubstituted, 1,1-disubstituted, cis-1,2-disubstituted epoxides and epoxides on cyclic systems epoxides to their corresponding diols. Because of its broad substrate specificity, this enzyme is thought to play a significant role in ameliorating epoxide toxicity. Reactions of detoxification typically decrease the hydrophobicity of a compound, resulting in a more polar and thereby excretable substance.
Soluble EH is only very distantly related to mEH and hydrates a wide range of epoxides not on cyclic systems. In contrast to the role played in the degradation of potential toxic epoxides by mEH, sEH is believed to play a role in the formation or degradation of endogenous chemical mediators. For instance, cytochrome P450 epoxygenase catalyzes NADPH-dependent enatioselective epoxidation of arachidonic acid to four optically active cis-epoxyeicosantrienoic acids (“EETs”) (Karara, A., et al., J. Biol. Chem., 264:19822-19877, (1989)). Soluble epoxide hydrolase has been shown in vivo to convert these compounds with regio- and enantiofacial specificity to the corresponding vic-dihydroxyeicosatrienoic acids (“DHETs”). Both liver and lung cytosolic fraction hydrolyzed 14,15-EET, 8,9-EET and 11,12-EET, in that order of preference. The 5,6 EET is hydrolyzed more slowly. Purified sEH selected 8S,9R- and 14R,15S-EET over their enantiomers as substrates. Studies have revealed that EETs and their corresponding DHETs exhibit a wide range of biological activities. Some of these activities include involvements in luteinizing hormone-releasing hormone, stimulation of luteinizing hormone release, inhibition of Na+/K+ ATPase, vasodilation of coronary artery, mobilization of Ca2+ and inhibition of platelet aggregation. Soluble epoxide hydrolase is believed to play a role in these biological activities by contributing to the regulation of the steady state levels of EETs and DHETs as well as other biologically active epoxides and diols.